In many electronic circuits, particularly in the field of communications technology, such as mobile radio technology, analogue continuous-time filter circuits are used. These frequently require the observance of cut-off frequencies in the range of tolerance of a few percent. This demand cannot be met without calibration, since the absolute values of integrated passive components (R and C) are subject to relatively large fluctuations related to technology. The same problem also arises for analogue time-delay elements if greater accuracy is demanded than can be provided by the technology.
One fundamental way of solving this problem is to trim the resistor and capacitor elements during production, although this has revealed that such a practice is very cost-intensive. In addition, the components produced and trimmed in this manner do not allow the equalization of temperature influences and drift during operation.
For this reason, filter circuits are manufactured using calibratable resistor or capacitor elements. The calibratability is usually produced by virtue of either a number of resistors that can be added in any combination being connected in series or parallel with the resistor element or a number of capacitors that can be added in any combination being connected in parallel with the capacitor element. The information about which resistors or capacitors need to be added in order to produce optimum filter properties is contained in a calibration signal that is generated in a calibration unit.
The calibration unit is usually part of the integrated circuit, which contains a reference RC element containing the same resistor and capacitor types as are used in the filter circuit that is to be calibrated. Before the filter circuit is started up, a voltage step at the level V is applied to the RC element, discharged beforehand, and the RC time constant is ascertained from the step response. FIG. 1 shows the profile v(t) for the step response in a voltage/time graph. The present level v(t) of the step response is compared with a fixed value v1. If this fixed value is exceeded at the time t1, the time measurement is stopped and the time that has elapsed is stored digitally. The time measurement is usually carried out using a binary counter clocked at a precise reference frequency fr.
The step response v(t) for an RC element is V×(1−exp(−t/(RC))), so that it holds that v1=V×(1−exp(−t1/(RC))).
The RC product can thus be clearly ascertained from V, v1 and the period t1 which can be measured using the frequency normal fr. It holds that RC=t1/(−1n(1−(v1/V))). The product RC is proportional to t1. For v1=V×(0.632), R×C=t1.
To calibrate integrated filters to the desired cut-off frequency fg, all R or all C in the filter need to be set to be proportional to 1/t1 in accordance with a factor K, since the frequency fg is proportional to 1/RC. Setting R or C is done using digitally programmable R or C elements, where b is the number of programming bits and the setting is possible in a particular range RCmin . . . RCmax.
The printed document U.S. Pat. No. 5,416,438 shows a filter arrangement as portrayed in principle above. This filter arrangement has, on a common semiconductor chip, a time-constant detector circuit and an active filter circuit that is controlled by the detector circuit. The detector circuit is made up of a time-constant circuit, containing a resistor and a capacitor, for generating a step-response signal, a reference voltage circuit for generating a reference voltage, a comparator, an AND gate, a four-bit pulse counter and an encoder for converting the count result into a calibration signal. The comparator is supplied with the step-response signal at its first input and with the generated reference voltage at its second input. The AND gate is supplied with the output signal from the comparator at its first output and with a reference clock at its second input. The AND gate continues to output reference clock pulses to the pulse counter only for as long as it receives an output signal from the comparator. As soon as the step-response signal outstrips the reference voltage, however, the output signal from the comparator changes to zero, which means that the AND gate no longer delivers reference clock pulses to the counter. The number of clock pulses counted by the counter is thus a measure of the RC time constant of the time-constant circuit if the reference voltage and the components are proportioned as described further above with reference to FIG. 1. The encoder supplies the active filter circuit with a three-bit calibration signal that can be used in two different variants to add three further resistors in series or parallel with the resistor used in the filter circuit.
An arrangement which is similar in principle is illustrated in the printed document U.S. Pat. No. 6,417,721 B1, where provision is made for the capacitor used in the active filter circuit to be able to have further capacitors connected in parallel with it under the control of the calibration signal. Further similar calibration arrangements and methods can be found in the printed documents US 2002/0067220 A1, U.S. Pat. No. 5,822,687 and U.S. Pat. No. 5,187,445.
In application, the calibratability of the active filter circuit is usually provided by calibratable capacitor elements, the calibration signal being a b-bit binary number and being applied to the calibratable capacitor elements in the active filter circuit via a b-bit parallel bus. The calibratable capacitor elements are usually binary-weighted switchable capacitor cascades. The calibration of the resistor elements in the active filter circuit as used in the first-mentioned printed document above is less customary on account of the corruption of the binary-weighted resistor values by the switches, and is relatively complex on account of the usually relatively large number of resistor elements.
A drawback of the prior art described above is that the reference RC element needs to have a correspondingly high RC time constant, which means that the time t1 can be measured at a resolution of >b bits for a given reference frequency fr. The magnitude of RC may necessitate an increased chip area, which increases the manufacturing costs for the circuit. In addition, a digital arithmetic and logic unit with multiplication and division is required, or at least a value table containing appropriately calculated correction values, in order to calculate the factor K. This is sometimes not wanted on purely analogue chips. Finally, interference and noise can corrupt the measurement of t1, since v(t) and v1 are compared only once by the comparator.